Lithium (Li)-Air electrochemical cells and batteries have some of the highest theoretical energy storage capacities in the energy storage technology field. Lithium-air batteries have a theoretical energy of around 3000 Wh/kg (watt hours per kilogram) which is approximately 10 times greater than that of lithium ion batteries. Additionally, Li-Air electrochemical cells are environmentally safe and have better stability than lithium ion batteries. As such, improvements in lithium-air batters are being actively developed.
Generally, Li-Air electrochemical cells include an anode (negative electrode) which includes lithium metal, an oxygen gas diffusive cathode (positive electrode) and an electrolyte having a lithium ion conductive medium positioned between the anode and the cathode. The oxidation of lithium at the anode and the reduction of oxygen at the cathode induce a current flow in the electrochemical cell.
However, during the discharge and recharge operation of the battery, there are several factors that decrease the specific energy of Li-Air batteries, such as Li dendrite growth and oxygen/water based degradation of the lithium metal. Dendrites are lithium deposits that form on the surfaces of the electrodes of the electrochemical cell which may continue to grow until they cause an internal short circuit, resulting in battery failure. Additionally, the growth of passive surface layers on the anode as a result of the presence of oxygen and water create resistance to lithium ion flow, thereby reducing the energy available from the Li-Air battery.
As a solution to the problems described above, lithium-air batteries that use non-aqueous as well as aqueous electrolytes have included an electrolyte separator that provides a barrier between the electrodes. The separator is typically a ceramic separator, such as a ceramic lithium ion conductor, that provides isolation of the lithium anode in a Li-Air battery to prevent degradation of the lithium anode and dendrite growth. While ceramic protective membranes are good ionic conductors, moisture barriers and Li dendrite suppressors, they suffer from the disadvantage of adding bulk to the battery. In addition, ceramic lithium ion conductors are not stable in contact with lithium, and as such, a polymer or gel electrolyte is required as an additional separator, adding complexity and cost to the battery.
Accordingly, there is a need in the art for a protective film for the lithium anode of a lithium-air electrochemical cell that provides lithium ion conductivity across the film, but which also prevents degradation of the lithium anode and inhibits lithium dendrite growth in the electrochemical cell.